Today researchers and imaging practitioners use any one of a number of non-invasive imaging techniques to produce an internal image. These techniques employ X-rays, magnetic resonance imaging (MRI), CAT scans and ultrasound. In other systems, optical imaging is used to produce optical images of an object. For example, bioluminescent imaging is a non-invasive technique for performing in vivo diagnostic studies on animal subjects in the areas of medical research, pathology and drug discovery and development. Bioluminescence is typically produced by cells that have been transfected with a luminescent reporter such as luciferase and can be used as a marker to differentiate a specific tissue type (e.g. a tumor), monitor physiological function, track the distribution of a therapeutic compound administered to the subject, or the progression of a disease. Fluorescence is another optical imaging technology that can be used to track cells or molecules in vivo. This technology has been demonstrated recently using genetically expressed reporters such as green fluorescent protein (GFP) and near infrared (NIR) dyes such as Cy5. Briefly, fluorescence is a molecular phenomenon in which a substance absorbs light of a particular wavelength and emits light of a longer wavelength. The absorption of light is referred to as the “excitation”, and the emission of longer wavelengths as the “emission”.
Each technique has advantages and disadvantages that make it useful for different imaging applications. Some techniques are well suited to provide spatial or anatomical information for internal parts while others are more suited to provide functional information for an activity of interest within an object being imaged.
Researchers are now finding it desirable to combine the output and strengths of multiple systems. However, the cost of each of these traditional imaging systems has become a barrier to such combination; and becomes even more prohibitive when contemplating multiple techniques at a single site. For instance, an MRI and computer tomography (CT) system can cost millions of dollars. In addition, most traditional imaging systems have practical complications that inhibit implementation in a multiple imaging system environment. MRI systems require that no ferrous metal be near the high power magnet during operation. Positron emission tomography (PET) centers are geographically limited to be close to a particle accelerator device that produces the short-lived radioisotopes used in the technique. Also, transferring an object to be imaged between different imaging systems would be difficult, such as it would be difficult to maintain the spatial accuracy provided by each system without compromise due to object transfer between the systems.
Given the foregoing, new systems for performing multiple modes of imaging would be desirable.